FIG. 1 is illustrative of prior art power generator excitation systems wherein the power source 1 may be internal to the power generator and consists of an exciter on the same shaft as the generator which includes an auxiliary three phase winding wherein power for the winding may be derived from the air gap flux of the generator. The potential for the winding is conventionally stepped down through the use of a power potential transformer 2 in order to meet low voltage generator field requirements. The transformer output is fed to a three phase rectifier bridge 3 to furnish the required DC power to the generator field winding 5 by way of standard regulation circuits 4 for regulating exciter field voltage and the generator terminal voltage.
In recent years rotating rectifier exciter systems have become popular and conventionally include AC generators with a rotating fused diode assembly along with a static voltage regulator for excitation control wherein the diodes are mounted on a wheel assembly attached to an extension of the generator shaft. In a conventional manner such excitation circuits for power generators are used to provide DC power to the field windings of the generator by rectifying the alternating current output of an exciter rotor. However, failure of a single diode in a shorted mode can result in significant hazardous conditions to system operation due to high voltages in the exciter field, high currents in the exciter armature and loss of excitation and control. Thus, rapid response of a detection and protection system is required in order to prevent subsequent damage to the exciter windings and the voltage regulator.
Conventional protective schemes currently in use require the rectifier to have redundant diodes in parallel along with fuses in series with each such diode. Alternatively, two diodes connected in series to carry the load current subsequent to a shorted diode failure of one of the diodes have been used. Moreover, doubling and quadrupling of such critical system components have also been proposed for redundant protection. However, doubling and quadrupling of such components clearly increases the probability of failure of one or more components as well as adding to product costs, increased losses, as well as increased ventilation requirements for cooling such components.
More recent applications have utilized bore pack rectifier assemblies where the rectifiers are mounted inside the generator shaft. Although such construction has been shown to reduce losses and increase reliability, such assemblies conventionally have no redundant diodes or fuses for protection in the event of a diode short. Accordingly, there is a continuing concern over the absence of a diode short detection and protection system, notwithstanding the expected high mean time between failures in such bore pack rectifier assemblies. In this regard, simulation tests at minimum excitation levels were conducted wherein the excitation system included a shorted diode. Such tests indicated that high currents in the armature are produced which resulted in an unbalanced rotating magnetic field which generated high alternating voltage in the exciter field windings. The high alternating voltage in turn was rectified into DC resulting in self excitation and loss of control.
I have discovered relatively simple detection and protection systems designed to be applied by direct attachment to the field leads of the exciter for detecting the AC voltage in the field windings due to the presence of a shorted diode in the excitation system. Such detection may thereafter be used for removal of all excitation from the generator field, as well as the field supply, and thus eliminate further damage to the excitation system. Moreover, since the protective systems proposed herein may be directly attached to the field leads of the exciter, they are suitable for direct application to existing generators equipped with DC field exciters, as well as bore pack or diode wheel rectifiers.
Moreover, the objects and advantages of my invention will be more completely understood and appreciated by the artisan carefully studying the following description of the presently preferred exemplary embodiments taken in conjunction with the accompanying drawings of which: